1. Field of the Invention
The present invention relates to a sheet transport apparatus which is configured to transport a sheet with rotation of a roller and an image forming system.
2. Description of the Related Art
There is conventionally known an image forming system which transports a sheet of paper placed on a tray along a transport path in a predetermined transport direction with rotation of a paper feed roller. In the image forming system, the paper transported with the rotation of the paper feed roller is supplied to a nip position at which the paper is nipped by a transport roller. The transport roller is positioned downstream of the paper feed roller in the transport direction. The paper supplied to the nip position of the transport roller is transported to an image-formation point positioned downstream of the transport roller with rotation of the transport roller.
In this type of image forming system, for example, the front end of the paper is positioned at the nip position by rotating the paper feed roller in a state that the transport roller is stopped or reversely rotated and thereby causing the paper to abut against the nip position of the transport roller. Thereafter, cueing of the paper to a target position positioned downstream of the nip position is performed by rotating the transport roller in a forward direction to transport the paper downstream in the transport direction. The “forward” direction referred herein is a rotation direction of the roller for transporting the paper downstream in the transport direction along the transport path. In the following description, the rotation direction of the roller in such a simple expression that the roller “rotates (is rotated)” means the “forward” direction.
Further, as a conventional system, there is known a system in which the paper is supplied to the nip position of the transport roller in a state that the transport roller is rotated in the forward direction (see Japanese Patent Application Laid-open No. 2005-335302). In this type of system, for example, in a case that image formation is performed continuously for a plurality of sheets of paper, the discharge of a preceding sheet of paper for which the image formation has been performed is performed concurrently with the cueing of a subsequent sheet of paper by rotating the transport roller in the forward direction.
In the system described in the above literature, a sensor which detects the front end of the paper is provided in the transport path between the paper feed roller and the transport roller. When the front end of the paper has been detected by the sensor in this system, a position count operation for the paper feed roller is started. In a case that a position count value for the paper feed roller has arrived at a predetermined value, a position count operation for the transport roller is started. Based on the position count value of the transport roller, the cueing of the paper to a target position is performed. The predetermined value corresponds to a rotation amount of the paper feed roller which is required to move the front end of the paper from a position at which the front end of the paper has been detected by the sensor to the nip position of the transport roller.
In this technique, however, it is more likely to cause an error between the count value and the actual paper transport amount, because the paper transport amount until the paper arrives at the nip position is indirectly measured by the position count operation of the paper feed roller. Therefore, it is easily to cause such a phenomenon that the paper does not reach the nip position when the position count value has arrived at the predetermined value. In this case, it is difficult to arrange the paper at the target position accurately.